Create a bunch of monsters and you'll probably notice a theme developing, intentional or not.
With Russell T Davies, it was anthropomorphic animals - he wanted to give young viewers something immediately relatable. I did think it maybe went a bit far when the season three trailer had a Cat People pilot, Pig Slaves and Judoon all in a minute. Since the Cat Pilot looked quite WWII as well, I was imagining a setting where different groups of animal people (like Catkind Fighter Pilots and berserker Pig Guys in boiler suits) were all fighting in a 40s style war. Could still be an interesting basis for an episode, I reckon.
With Steven Moffat, it's masks and unmoving faces. Nanogenes, check. Clockwork Robots, check. Weeping Angels, check. Skeletons in spacesuits, check. Smilers, check. Not the Atraxi or Prisoner Zero, but still, quite a list. Even the Silurians started wearing masks on his watch.
Mine would appear to be disguises. Sea Devils in coats, hats and scarves. Robots in Roman armour. Leanhaun Sidhe in human guise. Movellans. I avoid making it a constant, but since it's both a monster quirk and a plot point then if nothing else I'm being efficient.